The role of enterocyte defects in the pathogenesis of congenital diarrheal disorders.

Congenital diarrheal disorders are rare, often fatal, diseases that are difficult to diagnose (often requiring biopsies) and that manifest in the first few weeks of life as chronic diarrhea and the malabsorption of nutrients. The etiology of congenital diarrheal disorders is diverse, but several are associated with defects in the predominant intestinal epithelial cell type, enterocytes. These particular congenital diarrheal disorders (CDD(ENT)) include microvillus inclusion disease and congenital tufting enteropathy, and can feature in other diseases, such as hemophagocytic lymphohistiocytosis type 5 and trichohepatoenteric syndrome. Treatment options for most of these disorders are limited and an improved understanding of their molecular bases could help to drive the development of better therapies. Recently, mutations in genes that are involved in normal intestinal epithelial physiology have been associated with different CDD(ENT). Here, we review recent progress in understanding the cellular mechanisms of CDD(ENT). We highlight the potential of animal models and patient-specific stem-cell-based organoid cultures, as well as patient registries, to integrate basic and clinical research, with the aim of clarifying the pathogenesis of CDD(ENT) and expediting the discovery of novel therapeutic strategies.

Triacylglycerol-rich lipoproteins protect lipoprotein lipase from inactivation by ANGPTL3 and ANGPTL4.

Lipoprotein lipase (LPL) is important for clearance of triacylglycerols (TG) from plasma both as an enzyme and as a bridging factor between lipoproteins and receptors for endocytosis. The amount of LPL at the luminal side of the capillary endothelium determines to what extent lipids are taken up. Mechanisms to control both the activity of LPL and its transport to the endothelial sites are regulated, but poorly understood. Angiopoietin-like proteins (ANGPTLs) 3 and 4 are potential control proteins for LPL, but plasma concentrations of ANGPTLs do not correlate with plasma TG levels. We investigated the effects of recombinant human N-terminal (NT) ANGPTLs3 and 4 on LPL-mediated bridging of TG-rich lipoproteins to primary mouse hepatocytes and found that the NT-ANGPTLs, in concentrations sufficient to cause inactivation of LPL in vitro, were unable to prevent LPL-mediated lipoprotein uptake. We therefore investigated the effects of lipoproteins (chylomicrons, VLDL and LDL) on the inactivation of LPL in vitro by NT-ANGPTLs3 and 4 and found that LPL activity was protected by TG-rich lipoproteins. In vivo, postprandial TG protected LPL from inactivation by recombinant NT-ANGPTL4 injected to mice. We conclude that lipoprotein-bound LPL is stabilized against inactivation by ANGPTLs. The levels of ANGPTLs found in blood may not be sufficient to overcome this stabilization. Therefore it is likely that the prime site of action of ANGPTLs on LPL is in subendothelial compartments where TG-rich lipoprotein concentration is lower than in blood. This could explain why the plasma levels of TG and ANGPTLs do not correlate.

Chylomicrons promote intestinal absorption of lipopolysaccharides.

Recent data suggest that dietary fat promotes intestinal absorption of lipopolysaccharides (LPS) from the gut microflora, which might contribute to various inflammatory disorders. The mechanism of fat-induced LPS absorption is unclear, however. Intestinal-epithelial cells can internalize LPS from the apical surface and transport LPS to the Golgi. The Golgi complex also contains newly formed chylomicrons, the lipoproteins that transport dietary long-chain fat through mesenteric lymph and blood. Because LPS has affinity for chylomicrons, we hypothesized that chylomicron formation promotes LPS absorption. In agreement with our hypothesis, we found that CaCo-2 cells released more cell-associated LPS after incubation with oleic-acid (OA), a long-chain fatty acid that induces chylomicron formation, than with butyric acid (BA), a short-chain fatty acid that does not induce chylomicron formation. Moreover, the effect of OA was blocked by the inhibitor of chylomicron formation, Pluronic L-81. We also observed that intragastric triolein (TO) gavage was followed by increased plasma LPS, whereas gavage with tributyrin (TB), or TO plus Pluronic L-81, was not. Most intestinally absorbed LPS was present on chylomicron remnants (CM-R) in the blood. Chylomicron formation also promoted transport of LPS through mesenteric lymph nodes (MLN) and the production of TNFalpha mRNA in the MLN. Together, our data suggest that intestinal epithelial cells may release LPS on chylomicrons from cell-associated pools. Chylomicron-associated LPS may contribute to postprandial inflammatory responses or chronic diet-induced inflammation in chylomicron target tissues.

Development and physiological regulation of intestinal lipid absorption. II. Dietary lipid absorption, complex lipid synthesis, and the intracellular packaging and secretion of chylomicrons.

Research in dietary fat absorption has developed urgency because of the widely recognized epidemic of obesity in the United States. Despite its clinical importance, many controversies exist over some of the basic aspects of this process from the mechanisms of fatty acid uptake to the control of triacylglycerol export in chylomicrons. Recent advances have included the identification of a number of fatty acid transporters, the discovery of families of acyl-CoA synthetase long chains and acyltransferases, a physiological function for liver-fatty acid binding protein, and the characterization of the prechylomicron transport vesicle transporting chylomicrons from the endoplasmic reticulum to the Golgi.

Transport of vitamin E by differentiated Caco-2 cells.

In hepatocytes, vitamin E is secreted via the efflux pathway and is believed to associate with apolipoprotein B (apoB)-lipoproteins extracellularly. The molecular mechanisms involved in the uptake, intracellular trafficking, and secretion of dietary vitamin E by the intestinal cells are unknown. We observed that low concentrations of Tween-40 were better for the solubilization and delivery of vitamin E to differentiated Caco-2 cells, whereas high concentrations of Tween-40 and sera inhibited this uptake. Vitamin E uptake was initially rapid and then reached saturation. Subcellular localization revealed that vitamin E primarily accumulated in microsomal membranes. Oleic acid (OA) treatment, which induces chylomicron assembly and secretion, decreased microsomal membrane-bound vitamin E in a time-dependent manner. To study secretion, differentiated Caco-2 cells were pulse-labeled with vitamin E and chased in the presence and absence of OA. In the absence of OA, vitamin E was associated with intestinal high density lipoprotein (I-HDL), whereas OA-treated cells secreted vitamin E with I-HDL and chylomicrons. No extracellular transfer of vitamin E between these lipoproteins was observed. Glyburide, an antagonist of ABCA1, partially inhibited its secretion with I-HDL, whereas plasma HDL increased vitamin E efflux. An antagonist of microsomal triglyceride transfer protein, brefeldin A, and monensin specifically inhibited vitamin E secretion with chylomicrons. These studies indicate that vitamin E taken up by Caco-2 cells is stored in the microsomal membranes and secreted with chylomicrons and I-HDL. Transport via I-HDL might contribute to vitamin E absorption in patients with abetalipoproteinemia receiving large oral doses of the vitamin.

Chylomicron remnants regulate early growth response factor-1 in vascular smooth muscle cells.

Early growth response factor-1 (Egr-1) is a zinc-finger transcription factor that induces genes that promote atherosclerosis. The goal of the present study was to determine whether Egr-1 expression is modulated by atherogenic, triglyceride rich lipoproteins known as chylomicron remnants. Chylomicron remnants induced Egr-1 mRNA and protein expression in rat cultured vascular smooth muscle cells (VSMCs) and activated extracellular signal-regulated kinase (ERK) 1/2 in VSMCs. Further, chylomicron remnant-induced Egr-1 expression was inhibited by PD98059, a selective inhibitor of MAPK kinase (MEK), suggesting that the action of chylomicron remnants on Egr-1 was dependent on the ERK/MEK pathway. Chylomicron remnants also induced mRNA expression of the pro-inflammatory cytokines, IL-2 and IFN-gamma in VSMCs. We conclude that chylomicron remnants act as atherogenic lipoproteins via induction of Egr-1 expression and via cytokine-mediated inflammation.

Intestinal lipoprotein assembly.

PURPOSE OF REVIEW: The assembly of intestinal lipoproteins is critical for the transport of fat and fat-soluble vitamins. In this review we propose a nomenclature for these lipoproteins and have summarized recent data about their intracellular assembly and factors that modulate their secretion. RECENT FINDINGS: The assembly and secretion of intestinal lipoproteins increases with the augmented synthesis of apoB, apoAIV and lipids. Chylomicron assembly begins with the formation of primordial, phospholipid-rich particles in the membrane, and their conversion to large chylomicrons occurs in the lumen of the smooth endoplasmic reticulum. Chylomicrons are transported from the endoplasmic reticulum via specialized vesicles to the Golgi for secretion. The identification of genetic mutations in chylomicron retention disease indicates that Sar1b may play a critical role in this process. In addition to chylomicron assembly, intestinal cells have been shown to transport dietary cholesterol via apoB-independent pathways, such as efflux. SUMMARY: Understanding the mechanisms involved in the intracellular transport of chylomicrons and chylomicron-independent secretion pathways are expected to be the next frontiers in the field of intestinal lipoprotein assembly and secretion.

Postprandial chylomicrons: potent vehicles for transporting cholesterol from endogenous LDL+HDL and cell membranes to the liver via LCAT and CETP.

We examined whether postprandial (PP) chylomicrons (CMs) can serve as vehicles for transporting cholesterol from endogenous cholesterol-rich lipoprotein (LDL+HDL) fractions and cell membranes to the liver via lecithin:cholesterol acyltransferase (LCAT) and cholesteryl ester transfer protein (CETP) activities. During incubation of fresh fasting and PP plasma containing [(3)H]cholesteryl ester (CE)-labeled LDL+HDL, both CMs and VLDL served as acceptors of [(3)H]CE or cholesterol from LDL+HDL. The presence of CMs in PP plasma suppressed the ability of VLDL to accept [(3)H]CE from LDL+HDL. In reconstituted plasma containing an equivalent amount of triglycerides from isolated VLDL or CMs, a CM particle was about 40 times more potent than a VLDL particle in accepting [(3)H]CE or cholesterol from LDL+HDLs. When incubated with red blood cells (RBCs) as a source for cell membrane cholesterol, the cholesterol content of CMs, VLDL, LDL, and HDL in PP plasma increased by 485%, 74%, 13%, and 30%, respectively, via LCAT and CETP activities. The presence of CMs in plasma suppressed the ability of endogenous lipoproteins to accept cholesterol from RBCs. Our data suggest that PP CMs may play an important role in promoting reverse cholesterol transport in vivo by serving as the preferred ultimate vehicle for transporting cholesterol released from cell membranes to the liver via LCAT and CETP.

Induction of atherosclerosis by human chylomicron remnants: a hypothesis.

Epidemiologic studies have provided support for the association between delayed remnant removal and premature atherosclerosis. Triglyceride-rich particles such as chylomicrons and chylomicron remnants that carry dietary derived fats, may play a role in the early stages of developing arteriosclerosis. Currently research focuses on these lipoprotein classes seeking distinguishing factors that causes some lipoproteins to be atherogenic while others are not. Such lipoproteins could be involved in atherogenesis directly or indirectly. Direct involvement occurs by interaction of triglyceride-rich particles with the arterial wall, possibly affecting the artery wall by oxidative stress, direct endothelial toxicity by constituents such as lysophosphatidylcholine or oxysterols, induction of prothrombotic changes, stimulation of endothelial expression of cell adhesion molecules and direct interaction with circulating blood cells. Indirect involvement refers to the influence of triglyceride-rich lipoproteins on other lipoproteins on the composition of low density lipoprotein (LDL) and high density lipoprotein (HDL) particles. We propose that in individuals with delayed removal of chylomicron remnants, the prolonged exposure of areas of endothelium that have been partially activated by turbulent flow, to specific components of the remnants, results in the endothelial cells becoming further activated and able to bind monocytes. During or shortly after the transcytosis to the intima and transformation of monocytes to macrophages, the macrophages become engorged with remnant derived lipids and form the nidus of a fatty streak.

Chylomicron components activate duodenal vagal afferents via a cholecystokinin A receptor-mediated pathway to inhibit gastric motor function in the rat.

Nutrients in the intestine initiate changes in secretory and motor function of the gastrointestinal (GI) tract. The nature of the 'sensors' in the intestinal wall is not well characterized. Intestinal lipid stimulates the release of cholecystokinin (CCK) from mucosal entero-endocrine cells, and it is proposed that CCK activates CCK A receptors on vagal afferent nerve terminals. There is evidence that chylomicron components are involved in this lipid transduction pathway. The aim of the present study was to determine (1) the pathway mediating reflex inhibition of gastric motility and (2) activation of duodenal vagal afferents in response to chylomicrons. Mesenteric lymph was obtained from awake rats fitted with lymph fistulas during intestinal perfusion of lipid (Intralipid, 170 micromol h(-1), chylous lymph) or a dextrose and/or electrolyte solution (control lymph). Inhibition of gastric motility was measured manometrically in urethane-anaesthetized recipient rats in response to intra-arterial injection of lymph close to the upper GI tract. Chylous lymph was significantly more potent than control lymph in inhibiting gastric motility. Functional vagal deafferentation by perineural capsaicin or CCK A receptor antagonist (devazepide, 1 mg kg(-1), i.v.) significantly reduced chylous lymph-induced inhibition of gastric motility. The discharge of duodenal vagal afferent fibres was recorded from the dorsal abdominal vagus nerve in an in vitro preparation of the duodenum. Duodenal vagal afferent nerve fibre discharge was significantly increased by close-arterial injection of CCK (1-100 pmol) in 43 of 83 units tested. The discharge of 88% of CCK-responsive fibres was increased by close-arterial injection of chylous lymph; devazepide (100 microg, i.a.) abolished the afferent response to chylous lymph in 83% of these units. These data suggest that in the intestinal mucosa, chylomicrons or their products release endogenous CCK which activates CCK A receptors on vagal afferent nerve fibre terminals, which in turn initiate a vago-vagal reflex inhibition of gastric motor function.

Lipopolysaccharide (LPS)-binding protein mediates LPS detoxification by chylomicrons.

Chylomicrons have been shown to protect against endotoxin-induced lethality. LPS-binding protein (LBP) is involved in the inactivation of bacterial toxin by lipoproteins. The current study examined the interaction among LBP, chylomicrons, and bacterial toxin. LBP was demonstrated to associate with chylomicrons and enhance the amount of LPS binding to chylomicrons in a dose-dependent fashion. In addition, LBP accelerated LPS binding to chylomicrons. This LBP-induced interaction of LPS with chylomicrons prevented endotoxin toxicity, as demonstrated by reduced cytokine secretion by PBMC. When postprandial circulating concentrations of chylomicrons were compared with circulating levels of low density lipoprotein, very low density lipoprotein, and high density lipoprotein, chylomicrons exceeded the other lipoproteins in LPS-inactivating capacity. Furthermore, highly purified lipoteichoic acid, an immunostimulatory component of Gram-positive bacteria, was detoxified by incubation with LBP and chylomicrons. In conclusion, our results indicate that LBP associates with chylomicrons and enables chylomicrons to rapidly bind bacterial toxin, thereby preventing cell activation. Besides a role in the detoxification of bacterial toxin present in the circulation, we believe that LBP-chylomicron complexes may be part of a local defense mechanism of the intestine against translocated bacterial toxin.

Macrophages take up triacylglycerol-rich emulsions at a faster rate upon co-incubation with native and modified LDL: An investigation on the role of natural chylomicrons in atherosclerosis.

Chylomicrons play a role in atherosclerosis, however, because the mechanisms involved in the cell uptake of these particles are not fully understood, investigations were carried out using a radioactively labeled protein-free triacylglycerol-rich emulsion incubated with peritoneal macrophages obtained from normal and apoE-knockout mice. Experiments were done in the presence of substances that inhibit several endocytic processes: EDTA for low density lipoprotein receptor, fucoidan for scavenger receptor, cytochalasin B for phagocytosis, and a lipopolysaccharide for lipoprotein lipase. In addition, triacylglycerol-rich emulsions were also prepared in the presence of native or modified radioactively labeled low density lipoprotein particles that are known to accumulate in the arterial intima. Probucol was also used to prevent the possible role played by an antioxidant in triacylglycerol-rich emulsion uptake. We have shown that triacylglycerol-rich emulsion alone is taken up by a coated-pit-dependent mechanism, mediated by macrophage secretion of apolipoprotein E. Furthermore, native, aggregated, acetylated, and moderately macrophage-oxidized low density lipoprotein stimulate the uptake of a triacylglycerol-rich emulsion through several mechanisms such as an actin-dependent pathway, scavenger receptors, and lipolysis mediated by lipoprotein lipase. On the other hand, in spite of the interaction of low density lipoprotein forms with a triacylglycerol-rich emulsion, the cellular triacylglycerol-rich emulsion uptake is impaired by copper-oxidized low density lipoprotein, possibly due to its diminished affinity towards lipoprotein lipase. We have also shown that macrophages take up aggregated low density lipoprotein better than the acetylated or oxidized forms of low density lipoprotein.

Chylomicron remnants potentiate phenylephrine-induced contractions of rat aorta by an endothelium-dependent mechanism.

The effects of chylomicron remnants on endothelium-dependent contraction of rat aorta were studied in vitro. Chylomicron remnant particles were prepared in vivo from male Wistar rats and were incubated with aortic rings for 45 min before concentration contraction response curves were constructed to phenylephrine. Both native and oxidised chylomicron remnants significantly increased vessel sensitivity to this agonist. Oxidised chylomicron remnants also significantly increased the maximum response. This potentiation was abolished by endothelial removal, but was still evident in the presence of Nomega-nitro-L-arginine, with or without cyclo (D-alpha-aspartyl-L-prolyl-D-valyl-L-leucyl-D-tryptophyl) (BQ-123), indomethacin or superoxide dismutase. The study demonstrates, for the first time, that lipoprotein particles of dietary origin potentiate vascular contractions. This effect is endothelium-dependent, but is not due to inhibition of basal nitric oxide production or to stimulation of endothelin, superoxide or a cyclo-oxygenase-derived product.

Responses of celiac and cervical vagal afferents to infusions of lipids in the jejunum or ileum of the rat.

Multiunit celiac and single-unit cervical recordings of vagal afferents were performed before and during infusions of fatty acids, triglycerides, or saline into either the ileum or jejunum of the rat. In multiunit recordings, lipids increased activity of vagal afferents to a greater extent than saline. The greatest increases in vagal afferent activity resulted from infusions of linoleic acid, conjugated linoleic acid, or oleic acid. The triglycerides, corn oil or Intralipid, were less effective than the fatty acids in affecting vagal afferent activity. Ileal pretreatment with the hydrophobic surfactant Pluronic L-81 significantly attenuated the response of celiac vagal afferents to ileal infusion of linoleic acid. Single-unit recordings of cervical vagal afferents supported the multiunit data in showing lipid-induced increased vagal afferent activity in approximately 50% of ileal units sampled and 100% of a limited number of jejunal units sampled. These data demonstrate that free fatty acids can activate ileal and jejunal vagal afferents in the rat, and this effect can be attenuated by pretreatment with a chylomicron inhibitor. These data are consistent with the view that lipid-induced activation of vagal afferents could be a potential substrate for the inhibitory effects of intestinal lipids on gastrointestinal function, food intake, and body weight gain.

Metabolic adaptations to dietary fat malabsorption in chylomicron-deficient mice.

A mouse model of chylomicron deficiency was recently developed; these mice express a human apolipoprotein (apo) B transgene in the liver but do not synthesize any apoB in the intestine. Despite severe intestinal fat malabsorption, the mice maintain normal concentrations of plasma lipids and liver-derived apoB 100-containing lipoproteins. We investigated the metabolic mechanisms by which plasma lipid levels are kept normal. De novo lipogenesis (DNL) and cholesterogenesis were measured by mass isotopomer distribution analysis (MIDA). Plasma non-esterified fatty acid (NEFA) fluxes and hepatic re-esterification of labelled plasma NEFA were also measured. Hepatic and plasma triacylglycerol (TG) concentrations and plasma NEFA fluxes were not different between chylomicron-deficient mice and controls. The contribution from DNL to the hepatic TG pool was only modestly higher in chylomicron-deficient mice [12+/-2.1% (n=7) compared with 3.7+/-1.0% (n=9); means+/-S.E.M.], whereas cholesterogenesis was markedly elevated. The fractional contribution from plasma NEFA to hepatic TG was greatly elevated in the chylomicron-deficient animals (62% compared with 23%). Accordingly, 73% of hepatic TG was neither from DNL nor from plasma NEFA in controls, presumably reflecting prior contribution from chylomicron remnants, compared with only 26% in the chylomicron-deficient group. The long-term contribution from DNL to adipose fat stores reached approximately the same steady-state values (approximately 30%) in the two groups. Body fat accumulation was much lower in chylomicron-deficient animals; thus, whole-body absolute DNL was significantly lower. We conclude that plasma and hepatic TG pools and hepatic secretion of apoB-containing particles are maintained at normal levels in chylomicron-deficient mice, not by de novo fatty acid synthesis, but by more avid re-esterification of plasma NEFA, replacing the normally predominant contribution from chylomicrons, and that some dietary fat can be absorbed by apoB-independent mechanisms.

The role of apolipoprotein A-IV in food intake regulation.

Apolipoprotein (apo) A-IV is a glycoprotein synthesized by the human intestine. In rodents, both the small intestine and the liver secrete apo A-IV; the small intestine, however, is by far the major organ responsible for the circulating apo A-IV. Intestinal apo A-IV synthesis is markedly stimulated by fat absorption and appears not to be mediated by the uptake or reesterification of fatty acids to form triglycerides. Rather, it is the formation of chylomicrons that acts as a signal for the induction of intestinal apo A-IV synthesis. Intestinal apo A-IV synthesis is also enhanced by a factor from the ileum and that factor is probably peptide tyrosine-tyrosine (PYY). The inhibition of food intake by apo A-IV is probably mediated centrally. The stimulation of intestinal synthesis and secretion of apo A-IV by lipid absorption are rapid; thus, apo A-IV likely plays a role in the short-term regulation of food intake. Other evidence suggests that apo A-IV may also be involved in the long-term regulation of food intake and body weight. Chronic ingestion of a high fat diet blunts the intestinal apo A-IV response to lipid feeding and may explain why the chronic ingestion of a high fat diet predisposes both animals and humans to obesity.

[Lipoprotein receptors. Old acquaintances and newcomers]. / Les récepteurs des lipoprotéines. Vieilles connaissances et nouveaux venus.

Lipoprotein receptors are plasma membrane proteins of high affinity which interact with circulating lipoprotein particles. The well characterized LDL receptor continues to be analysed and some new findings on its intracellular mechanisms of action have emerged. New lipoprotein receptors have recently been described: the chylomicron remnant receptor or LDL-related protein (LRP), the lipolysis stimulated receptor (LSR), the very low density lipoprotein receptor (VLDLR), the HDL receptor (HDLR) and the scavenger receptor (SR). The molecular details of the receptors will facilitate the development of new therapeutic means to improve receptor-mediated clearance of lipoproteins.

Effect of the apolipoprotein C-II/C-III1 ratio on the capacity of purified milk lipoprotein lipase to hydrolyse triglycerides in monolayer vesicles.

The effect of the apolipoprotein C-II/C-III1 ratio on the capacity of purified bovine milk lipoprotein lipase to hydrolyse triglycerides was measured in a controlled model of pyrene-labeled nonanoyltriglycerides (1-2 ditetradecyl 3-pyrene nonanoyl glyceride) monolayer vesicles. Monolayer was composed of triglycerides, a non-hydrolysable phospholipid ether and cholesterol, a model system where the quality of the interface can be controlled. LPL released fatty acids from pyrene-triglycerides which were transferred from the lipoprotein structure to albumin. This transfer induces a decrease in the excimer production and in the excimer fluorescence intensity. Apolipoprotein C-II and C-III0 and C-III1 were purified from apolipoprotein VLDL. The 2 fragments, C-III1 A (peptide 1-40) and C-III1 B (peptide 41-79), were obtained after thrombin cleavage. Apolipoproteins C-III0 and C-III1 had a similar inhibitory effect on LPL. Inhibition with apo C-III0 or apo C-III1 was 85% of full LPL activity without inhibitor: Apo C-III1 B inhibited 62% of basal activity. It was 27% less effective than apo C-III1. Fragment C-III1 A did not inhibit LPL. The effect of change in both apo C-II (0-0.6 microM) and apo C-III1 (0-1.0 microM) on triglyceride hydrolysis shows the importance of the apo C-II/C-III1 ratio for the release of free fatty acids from triglycerides by LPL. The activating effect of apo C-II in the absence of the apo C-III inhibitor was maximal at 0.06 microM. No further activation was obtained between 0.06 and 0.30 microM. Higher concentrations decreased LPL activity. Apo C-III1 (0.1 microM) decreased the maximum activation by apo C-II from 0.0196 to 0.063 nmol/min/nmol LPL. Higher concentrations of apo C-III1 (0.1-0.5 microM) required higher apo C-II concentrations (0.30 microM instead of 0.06 microM) for maximal activation than when apo C-III1 was absent. The activity of the enzyme without apo C-II was decreased by 65% by 0.12 microM apo C-III1. Increasing the apo C-II/apo C-III1 ratio from 0.1 to 1, increased the activation of the enzyme by a given apo C-II concentration. Moreover, for a given apo C-II/C-III1 ratio, the LPL activation increased with the apo C-II concentration (between 0 and 0.010 microM), until a plateau was reached. This is important, as the change in the C-II/C-III1 ratio is not the only factor affecting LPL activity, and inhibition by apo C-III1 also depends on the overall quantity of apolipoproteins. Extrapolation of these results suggests that hyperlipoproteinemia seems to be more likely due to overproduction of VLDL, than to a decrease in lipoprotein lipase activity.